Rice and only rice. Stratified wild rice to be exact. Plant the appropriate amount per acre and leave yourself some areas unplanted to have some open water
Millet must be broadcast in inches of water or on dry mud. What happens with this is that the birds and deer get to it as it grows and by the time duck season rolls around and you flood your pond back out most of the millet has been eaten
Wild rice is the only native planting to North America. It is in a duck's DNA to love it and they will go crazy to get to it
Rice will also come back each year if your winter is cold enough. If the water stays around a freezing temperature for a lengthened time the stratification will take place and it will come back on its own. For hot winters like we have in NC it has to be planted each year
Stratified wild rice can be broadcast in the water and will sink to the bottom and grow. It will grow in up to 3 feet of water, so no need to drain/flood your pond like you have to do to have a good millet growth. It can also grow on mud where your pond is low
It is way past time to plant though, rice must be planted in the spring by April
It sure seems like the most logical choice- in the link I posted above they reference- Duck usage per day, it shows how many ducks per day can be supported by one acre of plantings- Rice far above all others listed. Invertebrates become increasingly important in addition to crops.